The present invention generally relates to control systems which move operators. The present invention more particularly relates to a pilot positioner and method for use in HVAC systems which provide accurate positioning of linear actuators in response to a control signal.
Actuators are generally used as operators to provide mechanical motion for various types of components in a system. In an HVAC system, actuators generally open and close pipes or intakes. The actuator is generally connected to a valve or damper and provides linear motion. By linearly moving the actuator, the valve or damper is moved. The valve or damper controls the amount of fluid traveling within a pipe or intake. A pneumatic operator such as a pneumatic actuator provides an output movement in response to a pneumatic input.
A pilot positioner is an apparatus which is generally connected to an actuator to provide more accurate positioning of the actuator. The positioner compares the output position with an input signal and readjusts the actuator in order to more accurately control the position of the actuator.
In the prior art, there are various ways to control valves and dampers with actuators. Totally pneumatic systems are found in the HVAC systems of commercial buildings throughout the United States. These systems rely on compressors in communication with pneumatic actuators and pneumatic pilot positioners such as the D9502 manufactured by Johnson Controls, Inc. The actuators are moved in response to various pneumatic controls as, for example, pneumatic thermostats. The pneumatic positioner receives a first position indicative signal from the actuator and a second pneumatic signal from the pneumatic control. The positioner outputs a third pneumatic signal to control the actuator. The actuator moves to a particular position in response to the third signal. The pneumatic positioner adjusts the position of the actuator by providing the third pneumatic signal in accordance with the first position indicative and second pneumatic signals.
Another type of system for controlling valves and dampers is a totally electronic system. These systems rely on electric actuators. In these systems, the actuator is generally a solenoid, motor, or other electric transducer which positions the damper or valve in accordance with electronic control signals. Over the last two years, digital computer controllers have become popular in new buildings because they are more efficient and more accurate. These systems appeared in the United States only recently, when electronic controllers became cost effective. A problem with electronic controllers is that they are not readily usable in pneumatic systems.
Recently, electronic controllers have been used with pneumatic systems. This hybrid of electric and pneumatic systems requires an electric to pneumatic interface. Electric to pneumatic interfaces are used so that the more accurate electronic controllers can be used with pneumatic systems. These prior art interfaces use electric to pneumatic transducers to convert the electrical signal to a pneumatic signal which is then applied to the pneumatic actuator. Due to the variation in pickup and full stroke pressure on the pneumatic actuators and also the load compliance of pneumatic actuators, many of these applications require the use of pneumatic pilot positioners to position the actuator. Thus, the hybrid system requires an electric to pneumatic interface and a pneumatic positioner in order to position the actuator.
At least three problems exist with respect to employing an electric to pneumatic interface. First, when installing an electric/pneumatic system or retrofitting a pneumatic system with an electronic controller, the pneumatic pipes must be opened in order to install the interfaces. Therefore, two trades are required to make the interconnection. Steam fitters or pipe fitters must be employed to connect the pneumatic system, and electricians must be employed to connect the electronic system. Bringing two subcontractors to a commercial installation is generally more expensive than bringing one subcontractor to a commercial installation. Second, pneumatic actuators are not able to provide the same positioning accuracy as electric actuators. Therefore, pneumatic positioners and interfaces are too inaccurate to exploit the superior accuracy of electronic control systems. Third, interface devices require additional mounting space in the control panel. The increased surface areas or footprints increase costs and critical area requirements for the controller as well as the entire system.
Thus, there is a need for an interface that can be readily interconnected to existing linear pneumatic actuators and which provides accurate positioning operations. Further, there is a need for an interface which can be readily connected to existing pneumatic actuators with minimal changes to existing pipes. Further still, there is a need for an interface for use with a pneumatic actuator which does not require the mounting of another device in the control panel.